Chromatic dispersion may result when performing optical coherence tomography (OCT) due to broadening and warping of the interference pattern, if the light is not accurately balanced between the reference and sample arm. Such dispersion causes a loss of resolution. Methods to compensate chromatic dispersion generally fall in two groups: techniques focused on the physical equalization of both arms through shaping of the corresponding dispersion relations, and techniques that rely on signal post-processing for the compensation of residual dispersion stemming from a physical imbalance. When chromatic dispersion is dependent on the scan depth, either because of the delay line, the physical configuration of the system, or the material properties of the tissue under study, physical equalization becomes more difficult. Software methods have been described for the compensation of chromatic dispersion that adapt to this situation. However, such software methods have disadvantages derived from their signal processing nature. In particular, their limited working range only allows for a moderate starting level of chromatic dispersion imbalance.
In ultrahigh resolution systems, the problem of depth-dependent chromatic dispersion is especially important, due to their low tolerance to dispersion mismatch. Additionally, systems based on integrated optics in technologies relying on strongly dispersive materials at the working wavelength (such as silicon at 1.3 μm), which try to adjust the working distance discretely by means of path-length switching schemes, also must deal with depth-dependent chromatic dispersion. Dealing with depth-dependent dispersion is also important in delay lines making use of any effect with dispersive properties, such as the thermo-optic effect in silicon at 1.3 μm.
A number of documents can be found in the patent literature regarding chromatic dispersion compensation. In particular, patent applications WO2005/117534, U.S. Pat. No. 5,994,690 and WO 2007/127395 A2 describe software-based dispersion compensation methods. In particular, application WO2005/117534 uses numerical methods for dispersion compensation; application U.S. Pat. No. 5,994,690 describes an algorithm using an autocorrelation function to correct image data, and application WO 2007/127395 A2 shows how to generate correction parameters for the compensation of dispersion.
An article by Guillermo Tearney et al. (“High-Speed Phase-and Group-Delay Scanning with a Grating-Based Phase Control Delay Line” Opt. Lett. 1997, 22 (27), pp. 1811-1813) describes a dispersion compensation system based on free-space optics and a diffraction network. However, this system can only address group velocity dispersion. The system requires discrete optics and cannot be integrated.
Patent application US2005/0058397 A1 describes a dispersion compensating system using three cascaded Mach-Zehnder interferometers to produce adjustable dispersion. Because of its interferometric working principle, its free spectral range (FSR) is limited, and there is a compromise between FSR and the maximum level of chromatic dispersion that can be obtained. The cited document describes how to use the disclosed invention to compensate dispersion in multichannel systems by choosing a FSR, which is an integer divider of the spectral separation between channels. Based on this configuration, the compensating device is described as achromatic. Although this denomination can be appropriate for multi-channel optical communication systems, the application to OCT would require an increase in FSR of several orders of magnitude relative to telecom parameters. Additionally, this system does not allow separate adjustment of group delay, group delay dispersion, and/or higher order dispersion terms.
Another patent application publication, US 2005/0018201 A1, describes a method and apparatus to increase the detection sensitivity in OCT and for low-coherence interferometry, but it does so through spectral division of signal bands.